The present invention relates to the fields of wave and optical communication switching and, more particularly, to switching devices using arrays of switches, and in particular microelectromechanical switches.
In fiber-optic communication systems, information is transmitted as a light or laser beam along a glass or plastic wire, known as a fiber. A significant amount of electronic communication and information transfer is effected through fiber-optic lines due to their much broader bandwidth and lower susceptibility to electromagnetic interference compared to conventional copper or metal wires. For example, much of the Internet and many long distance telephone communication networks are connected with fiber-optic lines. However, fast and efficient switching between optical fibers in a fiber-optic network has been difficult to achieve. Switches are needed to route signals at the backbone and gateway levels of these networks where one network connects with another, as well as at the sub-network level where data is being transported from its origin or to its destination. In particular, in a wavelength division multiplexed (WDM) optical fiber network, many channels, each occupying a distinct wavelength of light, are multiplexed within the same fiber. In a WDM network, optical multiplexers and demultiplexers are need to combine component wavelength signals into the main optical fiber path and/or separate the optical channels from the main fiber path.
Various prior art switching technologies have been employed in fiber-optic systems. For example, in electrical cross-connect (or electro-optical) switch technology, the optical signal is transformed into an electrical signal, a switching operation is performed with an electronic switch, and the electrical signal is then transformed back into the optical domain. Another prior art solution is to use an optical switch or cross-connect (OXC) capable of connecting and disconnecting optical fibers in the optical domain. Integrated optical OXC devices have been used for this purpose. These devices are constructed of a material, such as lithium niobate, generally in a planar waveguide configuration that allows switching action to take place between various input and output ports. More recently, optical switches based on emerging microelectromechanical system (or MEMS) technology have received considerable attention. MEMS, including micromechanical or micromachined systems, boast considerable promise for overcoming many of the limitations associated with alternative prior art fiber-optic switching technologies, especially those limitations relating to cost, efficiency, size, wavelength dependence, cross-talk, and signal attenuation. As used herein, the term microelectromechanical (MEMS) device is intended to embrace devices that are physically small and have at least one component produced using micromachining or other microfabrication techniques, and the term MEMS device includes microactuators, micromechanical devices, and micromachine devices.
Optical MEMS systems, also referred to as microoptoelectromechanical systems (MOEMS), use microoptical elements that reflect, diffract, refract, collimate, absorb, attenuate, or otherwise alter or modulate the properties and/or path of a light beam or signal. These types of optical switches can be made very compact and small, typically within the micrometer to millimeter range. The insertion loss of a MOEMS switch interface is comparable to alternative technologies, and occurs mainly at the entry port of the switch where light leaves a first optical fiber and at the exit port of the switch where light re-enters a second optical fiber. These losses are due to the enlargement of the beam dimensions in free space, and generally the greater the distance travelled by a light beam in free space, the greater the insertion loss of the switch will be (lenses may be used to help decrease this effect). The medium of a MOEMS switch is typically air, but a vacuum, inert gas, or other suitable fluid may also be used. The transmission of light within the switch medium, if kept relatively small, amounts for only a small portion of the overall attenuation. Additionally, the non-blocking medium of the switch ensures that no interference occurs when different light paths cross, enabling light beams to traverse without mutual effect, attenuation, or cross-talk: see generally, Hecht J., xe2x80x9cOptical switching promises cure for telecommunications logjamxe2x80x9d, Laser Focus World, page 69, (September 1998), the contents of which are incorporated herein by virtue of this reference.
For example, micromachined optical switches often use small mirrors that move to perform a switching operation. By actuating the mirror or moving element between a first position in which a light beam is allowed to pass unaffected by the mirror and a second mirror position in which the moving element reflects or interferes with the light beam, the path of an input light beam can be redirected into different outputs or otherwise interfered with. The use of mirrors, in particular, is advantageous since they operate independently of wavelength when reflecting an optical beam. However, MEMS switches or valves may also use other types of moving elements such as attenuators, filters, lenses, collimators, modulators, and absorbers to perform a desired switching operation. In general, to achieve low attenuation losses in a micromachined optical switch, the mirror or other optical element should be very smooth and of optical grade. In addition, the principle and means used to actuate the moving element of a MEMS device should be fast, simple, and provide reproducible and accurate alignment of the moving element. Furthermore, the actuator must be able to move that element by a sufficient amount to accomplish the switching task. An improved MEMS device capable of advantageously acting as such an optical switch is disclosed in applicant""s co-pending U.S. patent application Ser. No. 09/619,013, filed concurrently herewith and entitled xe2x80x9cMicroelectromechanical Device with Moving Elementxe2x80x9d, the contents of which are incorporated herein by reference.
To increase the capacity of fiber-optic communication networks, there is a growing desire and need to expand the number of fibers used in the network and/or the number of wavelength channels in a WDM fiber system. It is desirable and often necessary in these networks to have the capability to switch a given one of a plurality of inputs to a specific output. Consequently, the expansion of fiber-optic network capacity requires the use of high capacity switches capable of handling an increasing numbers of input and output ports. Such switches should be fast, efficient (i.e. have low losses), and compact. In addition, it is also desirable that the switching configuration be xe2x80x9cnon-blockingxe2x80x9d so that the switching of one input fiber to an output fiber does not interfere with the transmission of any other input fiber to any other output fiber.
Prior art optical cross-connects (also referred to as cross-bar configurations) typically perform the desired switching between input and output ports in a single two-dimensional rectangular array. For example, Lin in U.S. Pat. No. 5,960,132 describes an array of optical micromachined switches each comprising a reflective panel. An M-input by N-output cross-connect of the type taught by Lin, requires Mxc2x7N switching elements. Furthermore, for a uniformly spaced array of switching elements each separated by the distance d, the maximum possible free space switching distance between an input and output port is given as (M+N)xc3x97d. As a result, as the number of inputs and/or outputs in these optical cross-connects increases, the number of switches required to maintain full (non-blocking) switching flexibility rises rapidly, as does the size or footprint of the switching array. The insertion loss and cross-talk for certain input-output combinations in these two-dimensional cross-connects may also become unacceptably high due to a lengthening of the free space propagation distance for a light beam within the array and due to discrepancies and inaccuracies in the positioning of the micromachined switches. Furthermore, the micromachined switches may experience a considerable amount of friction during operation. For these reasons, a suitably compact and efficient optical switching device capable of switching between a large number of input and output ports has not been, heretofore, attained.
In U.S. Pat. No. 5,878,177 Karasan et al. disclose a switch architecture in which a layered switch fabric includes at least two switching layers. The layered optical cross-connect switches signals from an incoming set of optical fibers to an outgoing set of optical fibers. The input ports of each layer are fully connected to the outputs ports of that layer, i.e. any input can be switched to any output on a specific physical layer. The switching layers are not interconnected to one another, and so inputs on one layer cannot be switched to outputs on another layer, i.e. such connections are blocked. At least one switching layer receives a plurality of signals (e.g. separate WDM channels) from a common input optical fiber. To provide a more fully connected switching architecture, Karasan et al. further disclose an optical cross-connect having a two stage layered switch fabric. The first switching stage has a plurality of non-interconnected switching layers coupled to the incoming fiber trunks, and the second switching stage has a plurality of non-interconnected switching layers coupled to the outgoing fiber trunks. One output port of each switching layer of the first stage is coupled to one input port of each switching layer of the second stage via an interconnecting fiber trunk. The inclusion of a second stage thereby partly reduces the blocking resulting from a single stage cross-connect. Karasan et al. indicate that various types of switching elements may be incorporated into the switching configurations, including conventional mechanical, electro-optic, and microelectromechanical (MEMS) switches.
While the cross-connect switching configurations of Karasan et al. serve to reduce the size and dimensions of optical switching devices for high capacity networks, neither the single stage or double stage architecture provides for full connectability between inputs and outputs. More importantly, although Karasan et al. very generally suggest that MEMS switches can be incorporated into the disclosed configurations which they disclose, Karasan et al. do not teach or suggest any method of implementing the switching configurations with conventional MEMS switches. Although a three-dimensional configuration of conventional MEMS switches could technically be envisaged by dividing up portions of a large capacity two-dimensional configuration of conventional switches and simply stacking those portions one on top of another, the design difficulties in doing so compactly and efficiently are extensive. More specifically, conventional MEMS optical switches generally have optical switching elements etched within a substrate surface, so that the switching element or mirror is either disposed vertically with respect to the substrate or tilts (e.g. xe2x80x9cpops upxe2x80x9d) with respect to the substrate. The optical signal or input of the conventional MEMS switch travels parallel to the substrate surface, and the output of the switch is also directed parallel to the substrate surface. To perform a switching operation, the optical switching elements must be erected out of the substrate plane, and, as a result, small deviations in the position of the mirror from the desired angular position with respect to the substrate may significantly affect device operation accuracy. This problem is exasperated by any attempt to form large two-dimensional and particularly three-dimensional array structures with such conventional MEMS devices. Additional reliability concerns may also arise due to the high torsion and friction experienced by tilting switching elements. Thus, conventional MEMS optical switches inherently favor a two-dimensional configuration as switching only take places above the surface of a single two-dimensional physical switching layer, without the ability for signals to transit or switch between different layers. Furthermore, because of the erected configuration of the optical switching elements in conventional MEMS switches, it is difficult to place two-dimensional switching layers on top of each other and in addition these layers cannot be tightly spaced apart since sufficient separation must be provided for the switching operation on top of each layer to take place.
Consequently, there is a need for a compact, reliable, and low-loss MEMS based switching device that is suitable for high capacity networks having a large number of input and output ports. Preferably, the switches of the device should be relatively insensitive to switch positioning inaccuracies. It would also be desirable to provide an improved WDM multiplexer/demultiplexer for facilitating the use of such a switching device in a WDM fiber-optic network, as well as an improved switching configuration for providing broadcast or multi-cast capability.
In a principal aspect, the present invention provides a switching devices that receives a plurality of input signals and provides a plurality of output signals and has switches arranged in a two- or three-dimensional array configuration. At least two of the switches, which are preferably microelectromechanical MEMS switches, reside on distinct physical substrate layers in the switching device, and at least one of the signals travels through a penetrable zone of one of the physical substrate layers.
In one embodiment, the invention relates to a switching device for receiving a plurality of M input signals and providing a plurality of N output signals. Each input signal is directed along a path into the device and each output signal is directed along a path out of the switching device. The switching device has a plurality of switches arranged in a two-dimensional array, each of the switches being located at an intersection in which a projection of the path of one input signal meets a projection of the path of one output signal. At least two of the switches reside on distinct physical substrate layers in the switching device, and at least one of said signals travels through a penetrable zone of one of the physical substrate layers.
Similarly, in another embodiment the present invention provides a three-dimensional switching device having a plurality of P logical switching layers. Each of the logical layers receives a plurality of M input signals and provides a plurality of N output signals, each input signal being directed along a path into said layer and each output signal being directed along a path out of said layer. Each logical layer comprises a plurality of switches arranged in a two-dimensional array, each of the switches being located at an intersection in which a projection of the path of one input signal meets a projection of the path of one output signal. At least two of the switches in each logical layer reside on distinct physical substrate layers in the switching device. Also, at least one of the signals travels through a penetrable zone of one of the physical substrate layers.
Preferably, one switch is located at each intersection in which a projection of the path of one of the input signals meets a projection of the path of one of the output signals. Also preferably, each switch resides on one of the physical substrate layers near a penetrable zone of that physical substrate layer, so that in at least one operative position of the switch a signal input to or output by the switch passes through that penetrable zone. The signals may be optical signals and the switches may include mirrors. Most preferably, each switch is a microelectromechanical or MEMS switch and comprises a generally planar switching element disposed in parallel to the surface of the physical substrate layer on which the switch resides and an actuator operatively engageable with the switching element for moving the switching element between different positions in a plane parallel to the surface of the physical substrate layer. Other types of MEMS switches may also be used.
In another embodiment a switching configuration has a first three-dimensional switching device and a second three-dimensional switching device as above. The number of logical layers in the second switching device equals the number of output signals in each logical layer of the first switching device, and wherein one and only one output from each logical layer of the first switching device is received as an input to a logical layer of the second switching device. Preferably, the number of input and output signals in each logical layer of the second switching device equals the number of logical layers in the first switching device, and the logical layers of the second switching device are positioned orthogonally with respect to the logical layers of the first switching device so that the paths of the output signals from the first switching device are colinear with the paths of the input signals of the second switching device.
To, for example, provide a Clos switching configuration, the configuration may further include a third switching device with the number of logical layers in the third switching device equaling the number of output signals in each logical layer of the second switching device, and where one and only one output from each logical layer of the second switching device is received as an input to a logical layer of the third switching device. Preferably, the number of input and output signals in each logical layer of the second switching device equals the number of logical layers in the first switching device and the number of input and output signals in each logical layer of the third switching device equals the number of logical layers in the second switching device. Also preferably, the logical layers of the third switching device are positioned orthogonally with respect to the logical layers of the second switching device so that the paths of the output signals from the second switching device are colinear with the paths of the input signals of the third switching device.
In addition, a strictly non-blocking switching configuration may be provided, for example, with a first Clos switching configuration and a second switching configuration as above. For the first switching device in the first Clos switching configuration each logical layer includes a second path for each output signal out of the logical layer, and the second paths of the output signals are colinear with the paths of the input signals in that layer. For the third switching device in the second Clos switching configuration each logical layer includes a second path for each input signal into the logical layer, and the second paths of the input signals are colinear with the paths of the output signals in that layer. The first and second Clos switching configurations are positioned such that the second paths of the output signals from the first switching device of the first Clos switching configuration are colinear with the paths of the input signals of the first switching device of the second Clos switching configuration, and the paths of the output signals from the third switching device of the first Clos switching configuration are colinear with the second paths of the input signals of the third switching device of the second Clos switching configuration.
In another aspect, the present invention provides a method of fabricating the three-dimensional switching device above. The plurality of switches are fabricated on a main substrate surface, with the plurality of switches arranged in P rows, and the number of rows corresponding to the number of logical layers in the switching device. The plurality of switches on the main substrate surface are further divided into a plurality of sets of columns, with the columns in each set being uniformly spaced and each set of columns being separated from an adjacent column by a space equaling that of a single column. In this manner, each set of switches corresponds to the switches residing on one of the physical substrate layers. The method then further comprises separating the sets on the main substrate surface into the plurality of physical substrate layers, aligning the separated physical substrate layers to form the logical layers of the switching device, and bonding the physical substrate layers together.
In another aspect, the present invention provides an optical device having a first diffracting reflection grating having a plurality of diffraction elements on one side thereof, and a second diffracting reflection grating having a plurality of diffraction elements on one side thereof. The first and second diffracting reflection gratings are positioned in parallel with one another, separated by a distance w, so that the side of the first diffracting reflection grating having the diffraction elements opposes the side of the second diffracting reflection grating having the diffraction elements.
The optical device can be used as a wavelength division multiplexing (WDM) demultiplexer by directing a WDM signal at an initial input angle of incidence on to the diffraction elements of the first diffracting reflection grating so that the WDM signal is separated into a plurality of component wavelength signals. Each of the component wavelength signals is reflected, at different angles, by the first diffracting reflection grating onto the diffraction elements of the second diffracting reflection grating and thereafter further reflected by the second diffracting reflection grating so that the plurality of component wavelength signals are output by the optical device in parallel and uniformly spaced from one another.
Similarly, the optical device can be used as a wavelength division multiplexing (WDM) multiplexer by directing a plurality of parallel and uniformly spaced component wavelength signals at a common initial input angle of incidence on to the diffraction elements of the first diffracting reflection grating. Each component wavelength signal is reflected, at different angles, by the first diffracting reflection grating onto the diffraction elements of the second diffracting reflection grating and thereafter further reflected by the second diffracting reflection grating into a single WDM signal.
In yet another aspect of the present invention, a switching device suitable for multi-casting is provided. The switching device comprises a first set of inputs for receiving a plurality of inputs signals; a second set of inputs; a third set of inputs; a first set of outputs for providing a plurality of output signals; and a second set of outputs. A signal provided at one of the second set of outputs is directed to a splitter which divides the signal into a first split signal and a second split signal, the first split signal being directed to an input in the second set of inputs and the second split signal being directed to an input in the third set of inputs.
A signal received at one of the first set of inputs in the switching device may be multicasted by directing the multicast signal to one of the second set of outputs so that the multicast signal is received at one of the second set of inputs and at one of the third set of inputs. The multicast signal received at at least one of said one of the second set of inputs and said one of the third set of inputs is then directed to another of the second set of outputs so that the multicast signal is received at another of the second set of inputs and at another of the third set of inputs. These steps are repeated until the desired number of multicast signals are received at inputs of the second and third sets of inputs.